Techniques for the acoustic detection of fractures have been described in the art. These techniques involve the generation of an acoustic wave in the earth formation surrounding the borehole and detecting the degree of attenuation of an acoustic wave as it is strongly influenced by fractures in the path of the acoustic wave. Typically, the shear wave is recognized as not being transmitted through an open or fluid filled fracture. Hence, any crack or fissure in the earth formation in the path of a shear wave will strongly attenuate it. Known techniques for fracture detection thus involve transmitting an acoustic pulse into the formation and detecting the acoustic attenuation of the received waveform portion where the shear wave ought to be. A strong attenuation indicates the presence of a fracture and the orientation of the acoustic transmitter-receiver system relative to the borehole indicates the orientation of the fracture. Prior art patents which describe such transmissive attenuation type fracture detection techniques are the U.S. Pat. No. 2,943,694 to Goodman; U.S. Pat. No. 3,406,776 to Henry; U.S. Pat. No. 3,474,878 to Loren; U.S. Pat. No. 3,775,739 to Vogel and U.S. Pat. No. 3,794,976 to Mickler.
These prior art detection techniques involve reliance upon the transmissive influence by fractures, whose presence are deduced either from the absence of a shear signal or its very small amplitude when in view of the knowledge of the lithology of the formation, greater shear amplitudes would be expected. Since the receiver waveform from such fracture detection does not provide a positive indication of a signal representative of a fracture, its detection is more difficult.
Acoustic pulse-echo techniques have been described in the art to investigate boreholes; see, for example, U.S. Pat. No. 3,883,841 to Norel et al and U.S. Pat. No. 4,255,798 to Havira. These latter techniques involve the generation of an acoustic pulse to cause reflections from material interfaces in the path of the pulse. The reflections are then processed to evaluate the cement bond.
In the U.S. Pat. No. 3,502,169 to Chapman, a sonic borehole televiewer device is described to obtain a visual presentation of the wall of the borehole. An acoustic transmitter is used, operating in a frequency range of the order of about 2 MHz, to direct acoustic pulses at the borehole wall. The acoustic reflections from the wall are plotted as a function of azimuth, or circular scan to present a visual indication of wall fractures, cracks, as well as distinctions between hard and soft formations.
The U.S. Pat. No. 3,474,879 to Adair describes an acoustic pulse echo technique for scanning surface characteristics of a borehole with a rotationally mounted receiver-transmitter acoustic transducer. An acoustic beam generated by this transducer is directed at an angle relative to the borehole wall. The beam glances off with relatively little reflections in case of a smooth borehole wall, but when the beam is incident upon a wall discontinuity such as a cavern fracture or a rock interface, a detectable acoustic reflection is generated. The U.S. Pat. No. 3,464,513 to Roever teaches use of a similar system as in the Adair patent except that a plurality of stationary transducers are used to scan the periphery of the borehole wall.
The scanning of acoustic beams may be done mechanically as taught in the Adair patent or electronically as shown in the Roever patent or in U.S. Pat. No. 3,693,415 to Richard. Various techniques have been proposed to electronically steer an acoustic beam, see for example the U.S. Pat. No. 3,732,945 to Lavigne. An acoustic transceiver employing a flat array of transducers to enable the retrieval of a fish lost within a borehole is described in U.S. Pat. No. 3,935,338 to Aldrich et al.